1. Field of the Invention
The present invention relates to an improved composition for and method of stripping nickel and nickel alloys from metallic and non-metallic substrates. More particularly, the present invention relates to a stripping composition comprising at least one compound that is, or can form in solution, a zwitterion which acts as a complexing agent for the metal being dissolved as well as a buffer to control the pH of the stripping solution.
The present invention provides an electrical process for the removal of high-phosphorus electroless nickel plating from an article wherein the stripping solution itself is utilized in the application of a reverse current to said plated article. Stripping proceeds at the more rapid rate expected in the case of a nickel coating containing no phosphorus.
The present invention provides an electrical process for the removal from an article stripped of nickel of the residual smut that ensues from sulfide ion catalysis of the chemical stripping process. The electrical process according to the present invention utilizes the stripping solution itself at a low cost and without the use of hazardous chemicals.
2. Description of the Prior Art
Reclamation of fabricated articles by removal of defective nickel and nickel alloy plating is of significant economic importance to manufacturers. Accordingly, an inexpensive, easily controllable and enviromentally acceptable means of stripping unsatisfactory coatings is needed.
Methods and compositions suitable for removing nickel and nickel alloys from metallic and non-metallic substrates are well known in the art. Some of these stripping compositions incorporate strongly acidic systems which attack the substrate as well as the plating. More successful formulations are in the alkaline pH range; stripping solutions maintained at alkaline pH levels are particularly useful for stripping nickel from ferrous substrates which are rapidly attacked in acidic environments. To attain the proper alkaline range, prior formulations have had to utilize amines, cyanides or ammonia-containing compounds to complex the nickel.
The use of amines, cyanides or ammonia-containing compounds in the stripping solution according to the conventional method has certain drawbacks. Of particular importance in all stripping processes incorporating these compounds is the disposal of the spent stripping solution. Most disposal requirements include expensive ion exchange or oxidation methods which leave the operator with a residual nickel-free solution which still poses a high environmental liability as well as the additional expense of legal disposal. Furthermore, amines, cyanides and ammonia-containing compounds are all volatile at the economic operating temperatures of the stripping solution; essential ingredients are therefore lost through the volatilization process. Additionally, incorporation of cyanide compounds into the stripping solution composition introduces health hazards inherent in systems employing cyanide compounds.
The present invention is directed to a novel stripping composition which avoids the health hazards inherent in systems employing cyanides and which dramatically reduces the environmental problems involved in the disposal of the spent stripping bath by the elimination of the hazardous amines, cyanides and ammonia-containing compounds, while providing a stripping solution which may be used in the protective alkaline pH range.
Central to the present invention is the novel use of compounds that form zwitterions. A zwitterion, or dipolar ion, is an inner salt which is the product of a reaction between an acidic group and a basic group that are part of the same molecule, wherein the hydrogen attaches to the stronger base. Dipolar ions may be expected when a molecule contains an amine and an acid group if the amine is more basic than the acid's anion.
Zwitterions are found in nature in the form of proteins. Aminocarboxylic acid zwitterions commonly available on the market include betaalanine, alanine, glycine, valine, isoleucine, leucine, methionine and monosodium glutamate. The general structural formulas follow: ##STR1## Where: Q=NH.sub.2, OH, COOH, or H
n=0-6 PA0 m=0-6 PA0 (1) a complexing agent and buffer in the form of a zwitterion, PA0 (2) an oxidizing agent in the form of an organic nitro compound, PA0 (3) a pH-adjusting compound selected from the group consisting of a soluble salt that forms an alkaline solution when dissolved in water, potassium hydroxide and sodium hydroxide, to adjust the stripping solution to an operating pH range of 7 to 11, PA0 (4) a compound that forms the sulfide ion in solution to act as a catalyst in the chemical stripping process, and PA0 (5) when stripping nickel-iron coatings, a compound from the family of hydroxycarboxylic acids and/or polyphosphates and/or polyols to form an iron complexor. Monosaccharides may also be used to form an iron complexor. PA0 R=Na or K PA0 n=0-10 PA0 m=0 or 1; if m=0 the chain is terminated with H ##STR3## where: Q=COO.sup.(-) R.sup.(+) or H PA0 R=Na or K PA0 n=0-20 PA0 m=0 or 1; if m=0 the chain is terminated with H ##STR4## where: n=3-20
Surprisingly, zwitterions may be mixed with carbonates of sodium or potassium without any evolution of carbon dioxide, thereby creating the necessary conditions to strip metal in alkaline solution. The zwitterion acts as a complexing agent for the metal being dissolved as well as a buffer to control the pH.
The stripping solution described in the present invention works very quickly and with much greater operator safety than any stripping formulation disclosed in the prior art. The present invention provides a composition wherein the stripping process may be operated at significantly higher temperatures without production of hazardous fumes and without loss of essential ingredients through the volatilization that occurs in conventional compositions which utilize amines, cyanides and ammonia-containing compounds to complex the metal. A further advantage offered by the present composition is the option to increase the temperature of the bath to close to boiling point or to boiling point and thereby accelerate the stripping action and so increase productivity without chemical hazard to the operator.
An additional safety advantage of the present invention is that the composition is in a safe, solid, dry powder form that is not toxic to ship or store. Therefore, possible chemical hazards such as those brought about by, for example, accidental puncturing of the cannister are obviated.
According to the present invention, any one of several methods may be used to recover the dissolved nickel from the spent stripping solution, i.e., ion exchange, sulfide precipitation, pH precipitation, or preferably, direct electrochemical recovery using a diaphram cell. The residual solution is, unlike the solution prepared according to the conventional methods, free from hazardous amines, cyanides or ammonia-containing compounds and is acceptable for Public Treatment Works disposal.
The prior art does not disclose an efficient method for the removal of high-phosphorus electroless nickel plating. As stripping proceeds according to the conventional method, phosphorus from the plate forms a non-soluble film that halts further stripping. The present invention for the first time discloses a method for high-phosphorus electroless nickel stripping wherein an electrical current is applied to said plated article, stripping the phosphorus and causing it to go into solution. Stripping proceeds at the rapid rate expected in the case of a nickel coating containing no phosphorus.
The prior art does not disclose an efficient method for the removal of residual smut from stripped articles that ensues from sulfide ion catalysis of the chemical stripping process. The present invention provides an electrical process, utilizing the stripping solution itself at low cost to the consumer, which removes smut without use of hazardous chemicals such as cyanide or chromium compounds.